The distortion of the signal wave caused by the dispersion of a transmission optical fiber is a constraint factor of the long distance transmission. It is shown by a theoretical analysis that the higher the rate of the transmission system, the smaller the dispersion being permitted. In order to decrease the influence of the dispersion, an effective method currently used to solve the effect of the dispersion of the optical fiber to the system performance is to employ a dispersion compensating technology, that is, the dispersion compensating optical fiber is used to compensate the dispersion. Meanwhile, it is required that the dispersion compensating optical fiber has a dispersion profile which is an inversion of that of the communication optical fiber, that is, a negative dispersion slope.
For a dispersion compensating module on a transmission link, since it realization of the function of dispersion compensation compromise with the introduction of an insertion loss. The transmission performance of an optical communication system will be improved in several aspects by decreasing the insertion loss of the dispersion compensating module: decreasing the signal to noise ratio of the transmission system; simplifying the design of the optical amplifier module; decreasing the input power; and lowering the influence of the non-linear effect to transmission. Therefore, it is of great importance to decrease the insertion loss of the dispersion compensating module for the optimization of transmission performance, decrease in communication cost and simplification of system design. The insertion loss of the dispersion compensating module comprises both of the attenuation and the splice loss of the dispersion compensating optical fiber. In the case that the splice loss is determined, the insertion loss of the module depends on the attenuation of the optical fiber. The attenuation of the optical fiber Afiber is:Afiber=Att×L=Att×Dtotal/D=Dtotal/FOM wherein, Att is the attenuation coefficient of the optical fiber, L is the length of the optical fiber, Dtotal is the total dispersion of the module, D is the dispersion coefficient of the optical fiber, FOM is the Figure of Merit of the optical fiber, and:FOM=D/Att 
Therefore, the dispersion compensating optical fiber used for the dispersion compensating module should have not only a low attenuation coefficient but also a high Figure of Merit. At present, the Figure of Merit can reach a level of 200 ps/nm.dB, and the individual ones can reach 300 ps/nm.dB.
In the related patents, U.S. Pat. No. 5,361,319 has disclosed a general reflection index profile for a dispersion compensating optical fiber, however, the specifications of the attenuation (less than 1 dB/km) and the dispersion (less than −20 ps/nm.km) of which cannot satisfy the present communication transmission application, and its Figure of Merit is only 120 ps/nm.dB. In U.S. Pat. No. 2,002,0159731 A1, the Figure of Merit can reach 300 ps/nm.dB by using a method for optimizing the optical fiber reflection index profile and using a phosphorus doping material in an improved preform manufacturing process by chemical vapor deposition technology. However, this is at the cost of: the optical fiber has low negative dispersion (less than −180 ps/nm.km), and the operating wavelength is near the cut-off wavelength, thereby the optical fiber has a sensitive bend loss and operates unstably. The attenuation of the optical fiber also cannot be decreased easily (greater than 0.5 dB/km). Chinese Patent CN 1100273C has disclosed a refractive index profile of a dispersion compensating optical fiber and the properties that it can acquire, however, this patent does not relate to the attenuation and the Figure of Merit, so it can not estimate comprehensively the dispersion compensating optical fiber. Chinese Patent CN 1087432C has disclosed a preform manufacturing technology of a germanium dioxide and fluorine doped dispersion compensating optical fiber, however, the problems of the worse specifications of the attenuation (less than 1 dB/km) and the dispersion (less than −50 ps/nm.km) still exist in the said patent, and an important parameter, Figure of Merit, has not been related to.
Definitions of Some Terms in the Invention
Refractive index profile: the relationship between the glass refractive index of an optical fiber or a preform (including the core rod) of the optical fiber and its radius.
Sleeve tube: a thick wall high purity glass tube which meets the requirement of a certain cross section area.
RIT technology: inserting the core rod into the sleeve tube to form an optical fiber preform.
Oxygen and Silicon ratio (O/Si ratio): It is defined as the ratio of the total oxygen and (SiCl4+GeCl4) introduced into a substrate tube during deposition.
Relative refractive index: Δ%=[(ni2−n02)/2ni2]×100%, wherein ni is the optical fiber refractive index of the ith layer, n0 is the refractive index of a pure silica glass layer. ni is the largest refractive index indicated by Δ% in the optical fiber core region, unless otherwise stated. The radius of each sublayer is from the central line of the optical fiber to a farthest point from the central line of the said sublayer. The refractive index profile of the sublayer is the refractive index value of the said sublayer at the respective radial points.
Total dispersion is defined as an algebraic sum of the optical fiber waveguide dispersion and the material dispersion, in the optical fiber communication field, the total dispersion is referred to as optical fiber dispersion in unit of ps/nm.km.
Dispersion slope represents the dependence of the dispersion value upon the wavelength, being a slope of a curve depicted by taking the horizontal axis as the wavelength and the vertical axis as the dispersion value in the unit of ps/nm2.km. In a wavelength division multiplexing system, if the dispersion slope of the transmission link is large, than the deviation between each wavelengths becomes large, and the entire transmission performance will deteriorate.
Effective area Aeff is:Aeff=2π(∫E2rdr)2/(∫E4rdr)                wherein the integrating limit is from 0 to ∞, and E is an electrical field relating to propagation.        
PMD is the abbreviation of the polarization mode dispersion of the optical fiber.
MCVD: an improved chemical vapor deposition method
PCVD: a plasma chemical vapor deposition method
OVD: a vapor deposition method outside the tube
VAD: an axial vapor deposition method